One of the two men arrested, alleged republican militant Colin Duffy, has been charged with multiple murders over the past two decades, although he was never successfully convicted.
Northern Ireland police arrested two suspected Irish Republican Army militants Friday on suspicion of killing an off-duty prison officer, a rare slaying that has inspired political condemnation across Britain and Ireland.
The two men were arrested at their homes in Lurgan, a power base for IRA die-hards opposed to Northern Ireland peacemaking. Police said the elder suspect is Colin Duffy, the most prominent Irish republican in Lurgan, who has successfully defended himself against a series of murder charges dating back to 1993.
A gunman in a passing car shot prison guard David Black as he drove to work Thursday on Northern Ireland's main highway. Police said Mr. Black was hit by several bullets, then his car plunged off the road into a ravine. The attackers' getaway car was found, abandoned and burned out, in nearby Lurgan.
Black was the first prison officer to be killed in the British territory since 1993, the year before the major outlawed paramilitary groups on both sides began open-ended truces that blossomed slowly into a successful peace process.
None of Ireland's IRA splinter groups has claimed responsibility for the killing. But they have repeatedly threatened to kill prison staff as part of their campaign to rattle Northern Ireland with sporadic violence.
Over the past year, more than 40 jailed members of various IRA factions have waged a bitter protest inside Black's workplace, the high-security Maghaberry Prison midway between Lurgan and Belfast, and analysts say Black's killing could signal the start of a new effort to target off-duty guards. Unlike police and British soldiers, most guards are not supplied handguns for personal protection.
The prisoners seek an end to the guards' policy of strip-searching them for weapons parts, drugs and cell phones, all of which were smuggled into Maghaberry before the strip-searching policy began.
A Presbyterian minister who is a family friend of the Blacks, the Rev. Tom Greer, said they wanted to ensure that no extremist retaliated against the IRA's host Catholic community. Such tit-for-tat bloodshed was a hallmark of Northern Ireland's conflict until the 1990s cease-fires, but Protestant outlaws usually have not responded to IRA provocations over the past decade.
"His wife, Yvonne, is in pieces. She is broken," Mr. Greer said. "His children Kyle and Kyra, who are in the 17 to 21 age group, are stunned. They simply cannot take it in, but they are being very courageous."
Most IRA members loyal to the Provisional faction renounced violence and disarmed in 2005. But several breakaway groups, including the Real IRA faction that Mr. Duffy supports, continue to mount occasional bombings and shootings.
Duffy was charged with the 2009 Real IRA murders of two off-duty British soldiers, who were gunned down as they collected pizzas at the entrance of an army barracks. But he was acquitted earlier this year. He previously was charged with killing two policemen in Lurgan in 1997 when he was a reputed member of the Provisional IRA, but again was acquitted when the key witness withdrew her testimony.
He was convicted for the 1993 murder of a Protestant man, but it was overturned on appeal after the key witness against him was discovered to be a member of a Protestant paramilitary gang with ulterior motives for identifying Duffy as the killer.